1. Field of the Invention
The present invention relates to hydrophilic precipitated silicas of high pH and low silanol group density, to a process for preparing them and to their use, such as for defoamers, fillers and carriers.
2. Description of the Related Art
Hydrophilic precipitated silicas and processes for preparing them are known. Normally the silica is precipitated by simultaneous addition of an alkali metal silicate or alkaline earth metal silicate and an acidifying agent to an initial charge of water or silicate solution. Following precipitation the pH of the resulting suspension is adjusted to levels of between 2 and 4 by addition of acidifying agent, in order to prevent continuation of the reaction, and is then filtered, the filter product is dried and the dried silica is ground where appropriate. At the end of the production operation the resulting precipitated silicas then have a roughly neutral pH, i.e., pH values of between 6 and 8. Examples of such precipitations are found in EP 0 647 591 and EP 0 901 986. WO 2003014020 proposes precipitated silicas having a pH of at least 9.5 as support materials. WO 2003014020 teaches that the hydrophilic precipitated silicas with a pH of at least 9.5 must be rendered hydrophobic before they can be used as a component in defoamer formulations.
The use of hydrophilic and hydrophobic precipitated silicas in defoamer formulations is likewise known (Pigments Technical Bulletin 42, DEGUSSA, 06/1986). Utility in defoamer formulations imposes exacting requirements on the precipitated silicas. Thus they ought to be readily and effectively dispersible into the defoamer formulation and ought to lead to a rapid knockdown time (response time), complete knockdown (immediate effect) and long holddown (service life). Knockdown describes the ability of the defoamer to reduce the height of the foam immediately following addition, down to a defined height of the foam. Holddown characterizes the service life of the defoamer, i.e., the duration of its activity. Specifically a measurement is made of the time taken for the foam to regain a defined level. Alongside these the knockdown time characterizes the time taken to reach knockdown, relative to the foam maximum. Prior art precipitated silicas have unsatisfactory values for some if not all of the stated parameters.